Synthetic lubricants for aero gas turbines

ABSTRACT

AN AERO GAS TURBINE LUBRICANT HAVING OUTSTANDING THERMAL AND OXIDATION STABILITY, CORROSION RESISTANCE AND LOADCARRYING ABILITY AND GOOD LOW TEMPERATURE FLUIDITY IS BASED ON A HINDERED ESTER AND CONTAINS AN AROMATIC AMINE ANTIOXIDANT, A COPPER PASSIVATOR OF THE AZOLE OR SALICYLALDEHYDE SEMICARBAZONE TYPE AND A NEUTRAL PHOSPHATE AND CORROSION INHIBITOR AND A HYDROLYTIC STABILITY IMPROVER MAY ALSO BE INCLUDED N THE LUBRICANT. MAY ALSO BE INCLUDED IN THE LUBRICANT.

United States Patent ()ffice 3,790,481 SYNTHETIC LUBRICANTS FOR AERO GAS TURBINES Derek Clark Byford, Staines, and Peter Michael Blanchard, West Molesey, England, assignors to The British Petroleum Company Limited, Moor Lane, London, England No Drawing. Continuation of abandoned application Ser. No. 652,697, July 12, 1967. This application Apr. 30, 1971, Ser. No. 139,184

Int. Cl. Cm 1/10 US. Cl. 252-493 9 Claims ABSTRACT OF THE DISCLOSURE An aero gas turbine lubricant having outstanding thermal and oxidation stability, corrosion resistance and loadcarrying ability and good low temperature fluidity is based on a hindered ester and contains an aromatic amine antioxidant, :a copper passivator of the azole or salicylaldehyde semicarbazone type and a neutral phosphate and a dialkyl phosphite as load carrying additives. A lead corrosion inhibitor and a hydrolytic stability improver may also be included in the lubricant.

This application is a continuation of our application Ser. No. 652,697, filed July 12, 1967, now abandoned.

This invention relates to synthetic lubricants suitable for use under the severe conditions that exist in the operation of modern aero gas turbine engines. More specifically, the invention is concerned with a lubricant of this type that is based on a thermally stable ester and which contains an additive package principally designed to impart good high temperature anti-oxidation, anti-corrosion and load carrying properties to the basestock.

The problem of thermal stability in aero gas turbine lubricants can be satisfactorily handled by the use of certain hindered ester basestocks which generally also have good low temperature properties, being fluid in many cases at temperatures of 40 or below. A more difiicult problem that has to be faced however, is the problem of oxidation stability and resistance to corrosion which arises owing to the fact that the lubricants have to operate at high bulk oil temperatures (about 200 C.) in contact with air. These conditions have the effect of greatly accelerating the oxidative deterioration of the lubricant which generally results in an increase in its viscosity and acidity, and corrosion of, or formation of deposits on, metal surfaces. Excessive increase in viscosity can lead to a restricted flow of lubricant to the engine bearings resulting in inadequate lubrication on starting and/or inadequate cooling during engine running. Deterioration of engine component condition by excessive corrosion or deposition can lead to malfunctioning of the moving parts and excessive formation of oil insoluble materials can cause inadequate lubrication due to blockage of oil-Ways. It is therefore very desirable that a lubricant of this type should show no more than a slight tendency to increase in viscosity and acidity during service.

Another serious problem that has to be faced in lubricants of this type is the provision of adequate load-carrying ability. This problem arises owing to the fact that ester basestocks that are sufiiciently mobile to meet the low temperature requirements of lubricants of this type (for example to permit easy starting of the engines in extremely cold conditions) are very thin and lack body under the high temperature operating conditions. Various methods are used for assessing the load carrying ability of such lubricantsjfor example the well known IAE 3,790,481 Patented Feb. 5, 1974 gear machine. Government and engine manufacturers specifications usually specify minimum load carrying characteristics.

The principal object of the present invention is to provide a lubricating composition having outstanding load carrying ability and good thermal and oxidation stability, corrosion resistance and low temperature fluidity, whereby it is suitable in these respects for the lubrication of modern aero gas turbine engines.

According to the invention, there is provided a lubricating composition comprising an ester basestock consisting of a liquid neutral polyester that has been prepared by reacting together under esterification conditions and in one or more stages:

(i) an aliphatic monoand/or polyhydric alcohol having 5l5 preferably 5-10, carbon atoms per molecule and having no hydrogen atoms attached to any carbon atom in a 2 position with respect to any -OH group and (ii) an aliphatic monoand/or polycarboxylic acid having 2-14, preferably 312, carbon atoms per molecule,

. the basestock having dissolved therein:

(a) 0.5 to 7.5, preferably 1 to 5, percent wt. of an antioxidant of the aromatic amine type,

(b) 0.005 to 1.5, preferably 0.01 to 0.5, percent wt. of

a copper passivator,

(c) 0.5 to 5% wt. of a neutral organic phosphate of the formula (R O) PO Where the groups R are tolyl groups, phenyl groups, xylyl groups, or alkyl or cycloalkyl groups having up to 10 carbon atoms, and

(d) 0.001 to 0.5% wt. of a phosphorus compound of the formula (R O) P(O)H where the H is attached directly to the phosphorus atom and R is an alkyl or cycloalkyl group having up to 10 carbon atoms, e.g. butyl or cyclohexyl.

The additive concentrations in this specification are based on the ester basestock. It is to be understood that the composition may contain more than one member of each of the clases of ingredients specified.

THE BASE OIL The base oil is a hindered polyester of the type described above. By polyester is meant an ester having at least two ester linkages per molecule; it therefore includes diesters such as neopentyl glycol dipelargonate and di(2:2:4-trimethylpentyl) sebacate. The term neutral is used to mean a fully esterified product.

It is to be understood that in the esterification reaction described above there may be used more than one of any of the reactants mentioned, e.g. a mixture of monocarboxylic acids, and, in any case, the neutral ester product of the esterification reaction will sometimes consist of a mixture of diiferent ester molecules, so the expression polyester is to be construed in this light.

Examples of suitable acids and alcohols that may be usedin the preparation of the polyester are caprylic acid, capric acid, caproic acid, enanthic acid, pelargonic acid, valeric acid, pivalic acid, propionic acid, butyric acid, 2- ethylhexanoic acid, adipic acid, sebacic acid, azelaic acid, 2:2:4-trimethylpentanol, neopentyl alcohol, neopentyl glycol, trimethylolethane, trimethylolpropane, trimethylolbutane, pentaerythritol and di-pentaerythritol.

The most suitable polyesters are the esters of trimethylolpropane, trimethylolbutane, trimethylolethane, pentaerythritol and/ or di-pentaerythritol with one or more monocarboxylic acids having 3 to 10 carbon atoms, particularly one or more of those mentioned in the previous paragraph, and more complex esters, for example those prepared from trimethylolpropane, sebacic and/or azelaic acid, and one or more monocarboxylic acids having 3 to carbon atoms, particularly one or more of those mentioned in the previous paragraph. Most suitably, the trimethylolpropane and dicarboxylic acid are reacted in the molar ratio of 1:0.050.75, preferably 1:0.0750.4, the amount of monocarboxylic acid being sufficient to provide a carboxyl/hydroxyl balance in the reactants.

THE ANTIOXIDANT Suitable aromatic amine antioxidants include phenothiazine, iminodibenzyl, diphenylamine, phenyl-alphanaphthylamine and phenyl-beta-naphthlamine. Particularly suitable are the alkylated (e.g. C to C alkylated) aromatic amines, especially those of the formula where the R groups are alkyl groups having up to 14 carbon atoms (not necessarily the same at each occurrence in any given molecule), preferably octyl or nonyl groups. p,p'-Dioctyldiphenylamine is particularly effective. Other particularly suitable alkylated aromatic amines include the mono and di- C to C alkyl (e.g. the octyl and nonyl) phenothiazines, iminodibenzyls, diphenyl-p-phenylenediamines and phenylnaphthylamines. In some cases it may be desirable to use a mixture of antioxidants e.g. a mixture of a monoor di-alkyl diphenylamine, e.g. dioctyldiphenylamine, and one of the other amines specified e.g. a monoor di-alkyl phenyl naphthylamine.

THE COPPER PASSIVATOR Copper passivators are a well-known class of materials, the function of which is to reduce the extent to which copper is attacked by corrosive substances. The copper passivator used in the composition according to the invention must, of course, be soluble in the basestock. The effect of this additive is to reduce the corrosion on engine component materials when exposed to the lubricant for long periods at high temperatures and in the presence of air. The effectiveness of metal passivators can be measured by corrosion tests in which the protection of structural metals can be observed. Copper is the most critical metal in- 'volved in such tests and it has been found that if this metal can be effectively passivated, then the corrosion of the other metals present is negligibly small, except for lead. Suitable classes of copper passivator include:

('1) Those of the azole type such as imidazole, pyrazole, triazole and their derivatives, e.g. benzotriazole, methylbenzotriazole, ethylbenzotriazole, butylbenzotriazole, dodecylbenzotriazole, methylene bis benzotriazole and naphthotriazole.

(2) Salicylaldehyde semicarbazone and its C to C alkyl derivatives, e.g. methyl and isopropyl salicylaldehyde sernicarbazone.

(3) Condensation products of salicylaldehyde and hydrazine derivatives, and fatty acid salts of such condensation products. A particularly suitable hydrazine derivative is aminoguanidine and suitable fatty acids are those having 2 to 24 carbon atoms.

Particularly effective copper passivators are methylene bis-benzotriazole and salts of l-salicylalaminoguanidine and fatty acids having 13 to 18 carbon atoms, e.g. palmitic acid. Where the lubricant is intended for use in engines containing lead alloy components, it is desirable to include a lead corrosion inhibitor in the blend, usually at a concentration of 0.01 to 1.0, preferably 0.05 to 0.25 percent wt. Suitable lead corrosion inhibitors are C; to C alkyl gallates, neopentyl glycol disebacate, sebacic acid and quinizarin. Propyl gallate is preferred; it has no efiect on the other properties of the blend.

THE LOAD CARRYING ADDITIVES The compositions according to the invention incorporate a main load carrying additive and a much smaller quantity of a synergist or booster. The main load carrying additive is a neutral organic phosphate as specified under (c) above and is preferably used at a concentration of from 1.5 to 4.5 percent wt. Tri-tolyl phosphate is the preferred additive. The booster (see ((1) above) is used at a very much smaller concentration, preferably 0.01 to 0.1 percent wt. Preferred boosters are dibutyl phosphite, i.e. (C H O) P(O)H, and dicyclohexyl phosphite, i.e.

FURTHER OPTIONAL ADDITIVES If desired, the hydrolytic stability of the compositions according to the invention may be improved by the addition of 0.005 to 0.5, preferably 0.02 to 0.1, percent Wt. of a hydrolytic stability improver. Suitable ones are aliphatic or aliphatic/ aromatic amines having up to 30 carbon atoms, or hydroxyl derivatives thereof, preferably tertairy amines. The most suitable amines for this purpose are those of the general formula R (R )NR where R and R are alkyl groups having 1 to 4 carbon atoms and R is an alkaryl, or hydroxy-substituted alkaryl, group having up to 20 carbon atoms. A preferred compound of this type is 2:G-ditertiarybutyl-4-dimethylaminomethyl phenol.

The compositions according to the invention may also contain a very small proportion (up to 25 parts per million) of an anti-foam agent, e.g. a silicone.

EXAMPLES A number of examples of lubricating compositions according to the invention will now be described.

Three base oils were used in these compositions as follows.

Base oil P.A complex ester made by esterifying caprylic acid, 1zlzl-trimethylolpropane (TMP) and sebacic acid in the molar ratio of 10.7:4.23:1 and in the absence of a catalyst.

Base oil Q.A complex ester made by esterifying caprylic acid, THP and sebacic acid in the molar ratio of 28: 10:1 and using a catalyst.

Base oil R.-An ester made by esterifying pentaerythritol, enanthic acid and 2-ethylhexanoic acid in the molar ratio 1:3:1 and in the absence of a catalyst.

The additives used in the compositions were as follows:

DO DPA=p,p'-Dioctyldiphenylamine BTZ=Benzotriazole MBBTZ=Methylene bis-benzotriazole PG=Propyl gallate 'I'IP=Tritolyl phosphate DBP=Dibutyl phosphite DCHP=Di-cyclohexyl phosphite.

The proportions of the ingredients (parts by weight) used in the compositions are shown in Table 1 which also gives the kinematic viscosity in centistokes of the compositions at 210 F. and 40 F., the ASTM slope, the pour point F.) and the flash point (Cleveland Open Cup, F.).

TABLE 1 Composition A B O D E F Base oil P- O 100 100 100 100 R 100 DODPA 4.0 4.0 4.0 4.0 4.0 4.0 BTZ 0. 25 O. 25 0. 25 0. 25 0. 25 MBBT? 0. 02

G 0. 1 TT]? 4. 0 4. 0 4. 0 4. 0 4. 0 4. 0 BP 0.05 0. 05 0. O5 0. 05 0. 05 DCHP 0. 05

Viscosity at- 210 F., cs 7. 35 5. 32 5. 19 5. 39 5.40 5. 6 40 F., cs 23,200 9,370 16,240 9, 400 9,300 9,356 ASTM slope (210-100 F.)- 0. 669 0. 705 0. 735 0. 703 0. 702 0. 704 Pour point, F 65 '70 70 70 70 --70 Flash point, F 505 495 495 495 495 The compositions were evaluated for thermal stability, oxidation stability, corrosion resistance and load carrying by using tests of the type specificed in government and engine manufacturers specifications for lubricants for use in aero turbine engines suitable for supersonic jet aircraft.

Thermal stability test- This consisted in subjecting the lubricant to a temperature of 325 C. in an atmosphere of nitrogen for 24 hours and measuring the viscosity change on samples of the oil after 6, 12, 18 and 24 hours. A lubricant is considered to have adequate thermal stability for the above-mentioned purposes if its percent change in 210 F. kinematic viscosity in each case is within the limits to The results of the thermal stability tests are given in Table 2 which also gives the results obtained on a number of commercially available oils (X, Y and Z) that have been approved for use in jet aircraft engines. These oils have good thermal stability at 280 C. but are obviously seriously degraded at 325 C. These oils are based on diesters of dibasic acids and oxo alcohols thickened in various ways to attain a viscosity of about 7 cs. at 210 F.

Oxidation test This consisted in blowing air at a rate of 250 ml. per minute through a 50 ml. sample of the oil for 192 hours at each of various temperatures. The temperature at which an undesirable level of insoluble deposits is formed (0.05% Wt.) is noted.

Results on oils A and B are given in Table 3. These results are oustandingly good and very much better than can be achieved with oils of the type of X, Y and Z. The results on oils A and B are also better than the results obtained on three commercially available aero gas turbine oils Q, R and S that have been approved against an American engine manufacturers specification for a Type 2 aero turbine oil, i.e. an advanced type of aero turbine oil having very high anti-oxidation properties.

TABLE 3 Oil A B Q, R 3

Temperature, C 215 215 210 210 210 Corrosion test TABLE 4 Load carrying test The lubricating compositions were assessed by the wellknown IAE Gear Machine test in which a set of gears is sprayed with the oil being tested at certain elevated temperatures and the gears are run at certain speeds while a load is applied. The load when scuffing of the gears occurs is noted. Results on oils A, B, C, D, E and F and also on reference oil X are given in Table 5. Oils A to F meet or approach the very severe load-carrying requirements of the latest British specifications for advanced aero turbine lubricants for supersonic transport aircraft. These load carrying requirements are more severe than those in the previously mentioned American specification for a Type 2 oil against which oils Q, R and S have been approved.

It will be noted that the oils A, B, C, E and F have excellent load carrying ability, especially at 200 0., when compared with the Reference oil X.

What we claim is:

1. A lubricating composition comprising a lubricating amount of an ester basestock consisting of a liquid neutral polyester that has been prepared by reacting together under esterification conditions and in at least one stage:

(i) an aliphatic alcohol selected from the group consisting of monohydric alcohols, polyhydric alcohols, and mixtures thereof, said alcohol having 5-10 carbon atoms per molecule and having no hydrogen atoms attached to any carbon atom in a 2 position with respect to any OH group and (ii) an aliphatic acid selected from the group consisting of monocarboxylic acid, polycarboxylic acid and mixtures thereof, said acid having 3-12 carbon atoms per molecule,

the basestock having dissolved therein:

(a) 0.5 to 7.5% wt. of an aromatic amine antioxidant selected from the group consisting of an alkylated aromatic amine of the formula R C H NHC H R where the R groups are alkyl groups having up to 14 carbon atoms, and a blend of said alkylated aromatic amine with a mono-C to C alkyl phenyl naphthylamine or a ii-C to C alkyl phenyl naphthylamine,

(b) 0.005 to 1.5% wt. of a copper passivator selected from the groups consisting of methylene bis-benzotriazole, benzotriazole, methyl benzotriazole, ethyl benzotriazole, butyl benzotriazole, dodecylbenzotriazole and naphthotriazole,

(c) 0.5 to 5% Wt. of a neutral organic phosphate of the formula (R O) PO where the groups R are selected from the group consisting of tolyl groups, phenyl groups, xylyl groups, alkyl groups having up to 10 carbon atoms, and cycloalkyl groups having up to 10 carbon atoms, and

(d) 0.001 to 0.5% wt. of a phosphorous compound of the formula (R O) P(O)H where the H is attached directly to the phosphorous atom and R is selected from the group consisting of alkyl groups having up to 10 carbon atoms or cycloalkyl groups having up to 10 carbon atoms.

2. A lubricating composition according to claim 1, in

which the polyester is an ester of an alcohol selected from the group consisting of trimethylolpropane, trimethylolethane, trimethylolbutane', pentaerythritol, dipentaerythritol and mixtures thereof with at least one monocarboxylic acid having 3 to 10 carbon atoms.

3. A lubricating composition according to claim 1, in which the polyester is a complex ester prepared from (a) trimethylolpropane, (b) a dibasic acid selected from the group consisting of sebacic acid, azelaic acid and mixtures of said acids and (c) at least one monocarboxylic acid having 3 to 10 carbon atoms.

4. A lubricating composition according to claim 1, which also contains 0.01 to 1% wt. of a lead corrosion inhibitor.

5. A lubricating composition according to claim '4, in which the lead corrosion inhibitor is selected from the group consisting of a C to C alkyl gallate, neopentyl glycol disebacate, sebacic acid and quinizarin.

6. A lubricating composition according to claim 1, which also contains 0.005 to 0.5 wt. of a hydrolytic stability improver. a a

7. A lubricating composition according to claim 6, in which the hydrolytic stability improver is an amine having the general formula R(R )NR where R and R are alkyl groups having 1 to 4 carbon atomsand R5 is an alkaryl or hydroxy-substituted alkaryl group'having up to 20 carbon atoms.

8. A lubricating composition comprising a lubricating amount of an ester basestock consisting ofa liquidneutral polyester prepared by reacting (a) one molar proportion of trimethylolpropane with (b) 0.075 to 0.4 molar proportions of a compound selected from the group consisting of sebacic and azelaic acid, and mixtures of said acids and (c) at least one monocarboxylic acid having 3 to carbon atoms to provide a hydroxyl/carboxyl balance in the reactants, the said polyester having dissolved there- 1n:

(a) 1 to 5% Wt. of dioctyldiphenylamine,

(b) 0.01 to 0.5% wt. of benzotriazole or methylene bis-benzotria'zole,

(c) l.5 to 4.5% wt. of tritolyl phosphate and (d) 0.01 to 0.1% Wt. of dibutyl phosphite or dicyclohexyl phosphite.

9. A lubricating composition comprising a lubricating amount of an ester basestock consisting of a liquid neutral polyester prepared by reacting one molar proportion of pentaerythritol with four molar proportions of a monocarboxylic acid having 3 to 10 carbon atoms, the said polyester having dissolved therein:

(a) 1 to 5% Wt. of dioctyldiphenylamine, Y (b) 0.01 to 0.5% wt. of benzotriazole or methylene bis-benzotriazole, (c) 1.5 to 4.5% wt. of tritolyl phosphate and (d') 0.01 to 0.5% wt. of dibutyl phosphite or dicyclohexyl phosphite.

References Cited UNITED STATES PATENTS 3,321,401 5/1967 Ford et al 252-49.9 X 3,321,402 5/1967 Dadura et al 25256 S X 3,236,774 2/1 966 Thompson etal. 25.246.7

FOREIGN PATENTS 986,068 3/1965 Great Britain.

OTHER REFERENCES Gunderson et al.:' Synthetic Lubricants, Reinhold Pub. Co. (1962), p. 210. DANIEL E. WYMAN, Primary Examiner W. CANNON, Assistant Examiner 222 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,790,431 Dated February 5, 1974 Inv ntor(a)Derek Clark Bvford, Peter'Michael Blanchard It is certified that eri'or appears in the above-idem:lfied patent and that said Letters Patent are hereby corrected as shown below:

Column 5, Table 4, under the heading designation "Oil B",

sixth entry from the top, change "+0.10" to Signed and sealed this 21st day of May 197A.

(SEAL) Attest:

EDWARD 1-i.FLE1GHiR,JH-. I v i U. I-LARSHALL DAM-I Attesting Officer Commissioner of Patents 

